Strain coupling mechanisms and elastic relaxation associated with spin state transitions in LaCoO₃

Abstract

Advantage is taken of the wealth of experimental data relating to the evolution with temperature of spin states of Co(3+) in LaCoO₃ in order to undertake a detailed investigation of the mechanisms by which changes in electronic structure can influence strain, and elastic and anelastic relaxations in perovskites. The macroscopic strain accompanying changes in the spin state in LaCoO₃ is predominantly a volume strain arising simply from the change in effective ionic radius of the Co(3+) ions. This acts to renormalize the octahedral tilting transition temperature in a manner that is easily understood in terms of coupling between the tilt and spin order parameters. Results from resonant ultrasound spectroscopy at high frequencies (0.1-1.5 MHz) reveal stiffening of the shear modulus which scales qualitatively with a spin order parameter defined in terms of changing Co-O bond lengths. From this finding, in combination with results from dynamic mechanical analysis at low frequencies (0.1-50 Hz) and data from the literature, four distinctive anelastic relaxation mechanisms are identified. The relaxation times of these are displayed on an anelasticity map and are tentatively related to spin-spin relaxation, spin-lattice relaxation, migration of twin walls and migration of magnetic polarons. The effective activation energy for the freezing of twin wall motion below ~590 K at low frequencies was found to be 182 ± 21 kJ mol(-1) (1.9 ± 0.2 eV) which is attributed to pinning by pairs of oxygen vacancies, though the local mechanisms appear to have a spread of relaxation times. It seems inevitable that twin walls due to octahedral tilting must have quite different characteristics from the matrix in terms of local spin configurations of Co(3+). A hysteresis in the elastic properties at high temperatures further emphasizes the importance of oxygen content in controlling the properties of LaCoO₃.

The role of defects in microwave and conventionally synthesized LaCoO3 perovskite

“…LCO [19], and is now confirmed in the MW synthesized species. This indicates that the concomitant coupling of magnetic and dielectric properties via the crystal lattice is an inherent and intrinsic property of the LCO perovskite phase, which agrees well with recent findings of changes in the crystal lattice strain across Ts1 [60].…”

Acoustic attenuation due to transformation twins in CaCl2: Analogue behaviour for stishovite

“…Regularly spaced background peaks at high temperatures which do not change frequency to any great extent with temperature are seen in all spectra collected with the high temperature instrument (e.g. Thomson et al, 2010;Zhang et al, 2011) and are due to the alumina buffer rods. At least some of the noisy background at very low temperatures arises from somewhere in the sample holder rather than from the sample itself.…”

Elastic and anelastic anomalies due to spin-state transitions in orthorhombic perovskite from isoelectronic behavior of Co3+ and Fe2+

“…The present work builds also on a recent study of elastic and anelastic properties of rhombohedral (R3c) LaCoO 3 measured using resonant ultrasound spectroscopy (RUS) and dynamical mechanical analysis (DMA) (Zhang et al 2011). The shear modulus stiffens with falling temperature in a manner that follows qualitatively an empirical spin order parameter for the sequence IS → LS+HS → LS.…”

“…Arrhenius form of anelasticity map for thermally activated relaxational processes in LaCoO 3 (from Zhang et al 2011). Each line represent the condition ωτ = 1 for a given relaxational mechanism.…”

“…In addition, however, LaCoO 3 shows four distinct acoustic loss mechanisms. These are shown in the Arrhenius form of anelasticity map in Figure 1 (Zhang et al 2011;citing Senaris-Rodriguez and Goodenough 1995;Murata et al 1999;Zhang et al 2005;Naing et al 2006;Hoch et al 2009), with each line defining the condition ωτ = 1 in the Debye equation for anelastic loss, where ω (=2πf for f in Hz) is the applied frequency and τ is the relaxation time. Spin-state changes on their own or coupled to lattice strain occur with relaxation times that are far too short to be relevant for the timescales of seismic waves, but twin wall motion and the mobility of defects, which couple to a local change in spin state have at least the potential to cause attenuation of seismic waves.…”